1. Field of the Invention
The present invention relates generally to a plasma etching method. More particularly, the invention relates to a plasma etching method employing a parallel plate type plasma etching device whereby an anisotropic etching is attained.
2. Description of Relevant Art
In the manufacture of a large-scale integrated circuit such as an LSI or ultra LSI, it is necessary that a semiconductor material, a semiconductor film used for the production of semiconductor devices, an insulation film, or a metallic film be etched exactly following a fine pattern. At present, in the manufacture of LSI's or the like, there is employed a plasma etching method wherein a gas is activated by utilization of an electric discharge and the surface of a material to be etched is etched by the activated gas following a pattern.
Generally employed as a device for performing the aforesaid plasma etching is a so-called cylindrical type etching device wherein a material to be etched is placed in a cylindrical reaction tube made of quartz, an electrode is wound around the quartz tube in the form of a coil or an electrode plate is fixed around such tube, and the gas in the reaction chamber is plasmized by application of a high frequency to thereby etch the surface of the material to be etched.
In the etching employing the aforesaid cylindrical etching device, however, the ion bombardment contributes only at a very low energy to the material to be etched, and a radical etching predominates. Therefore, as shown for example in FIG. 8 of the accompanying drawings, if a polysilicon film 2 is formed on a base 1 made of SiO.sub.2, and then a photoresist mask 3 is applied onto the surface of the film 2 and the film 2 is etched, there is performed an isotropic etching involving a side etching as shown in FIG. 9. Such an isotropic etching makes it impossible to attain an exact etching along the mask pattern, and thus there is a limit with respect to attainable fineness.
To avoid the aforesaid drawback, it is desirable to effect a so-called anisotropic etching involving a perpendicular etching of the polysilicon film 2 with respect to the base 1, as shown in FIG. 10. As a device for carrying out such an anisotropic etching, there is known a parallel-plate type plasma etching device such as shown in FIG. 11, wherein a plate-like ground electrode 5 and a counter electrode 6 (having an area width substantially equal to that of a wafer to be etched, as shown in FIG. 11) are disposed within a reaction chamber in spaced parallel relation in the vertical direction, a semiconductor wafer is placed on the lower ground electrode 5, while the spacing between the electrodes 5 and 6 is set at 30 to 100 mm in consideration of the uniformity of etching, etc., and a high-frequency voltage is applied between the electrodes, whereby an etching gas dissociates in the discharge zone to generate ions, electrons and radicals. Because the mobility of electrons and that of ions are different, there occurs a deviated electric field near the electrode, where a negative potential acts against the discharge zone, so that reactive ions are accelerated onto the electrode, thus permitting performance of an etching having directionality. However, under some particular gas and reaction conditions, etching is carried out mainly by a radical as a chemical reaction source, and an anisotropic etching is not always attained. Therefore, such known plasma etching device is not considered satisfactory at present, and requires a higher integration.
As another method for carrying out an anisotropic etching, there is known a reactive ion etching method wherein etching is performed by producing a bias voltage (cathode drop voltage) positively. However, such method is disadvantageous in that etching must be conducted at an extremely high vacuum (substantially 10 Pa), thus requiring the provision of a suitable apparatus therefor, and the etching speed is low.
In this respect, it is also known that such reactive ion etching is carried out at a pressure no higher than 100 mTorr and in which there does occur a physical etching resulting in an anisotropic character of etch, whereas a plasma etching, with which the present invention is concerned, is carried out a pressure higher than 100 mTorr and no higher than 10 Torr (100 mTorr .ltoreq.P&lt;10 Torr), and in which there does occur a chemical etching resulting in an isotropic character of etch.
The present invention effectively overcomes the above-mentioned drawbacks attendant conventional plasma etching methods.